Hi Rolf,
My shop is a detached building and has insulated walls and roof. I use an electric baseboard unit of about 2.5 KW. These are cheap, easy to install, and about as safe as you can get. On the coldest nights it will keep the shop at 50 DegF (0 - 10 outside). To supplement this I also have a propane unit that screws directly into the top of a 20 lb. propane tank.
One thing to keep in mind is that a 5 gallon bucket or 55 gallon drum of resin takes forever to reach ambient temperature when you turn on the heat in a cold shop. I used to try saving money by keeping the shop cold until it was time to do a layup but this turned out to be not very practical. Then, I tried hiding containers of resin in odd corners of the house and my wife got really pissed at me.

Kevin